1. Field of the Invention
The present invention relates to a dispenser unit for a non-gaseous flowable material, and in particular to a dispenser unit suitable for dispensing metered quantities of a material by measuring its flow.
2. Description of the Prior Art
Dispenser units which meter the amount of material dispensed by measuring the flow of the material from the unit are well known. A common method of measuring the flow is to measure the pressure difference developed across a flow restriction, such as an outlet of the dispenser, as material flows therethrough.
One known dispenser unit which operates according to this method is described in U.S. Pat. No. 4,922,832, and has a holder for pressurized fluid connected to a dispensing nozzle. A pressure transducer is provided to sense the fluid flow by measuring the pressure drop across the nozzle as the fluid flows. A valve is also provided which is controllable in response to the sensed flow to deliver a metered amount of the fluid through the nozzle. In order to measure very small flows, for example in the range of 0.005-20 ml/min, or fluctuations therein, the pressure transducer must be made extremely sensitive which is expensive and difficult to do. Moreover, the pressure transducer must be made very stable since drift in the baseline output with time can lead to inaccurate flow readings. Frequent calibration of the pressure transducer is often required to alleviate this problem, which is time consuming.
It is an object of the present invention is to provide a dispensing unit in which the problems associated with performing the pressure measurements to provide the flow information in known dispenser units are alleviated.
The above object is achieved in accordance with the principles of the present invention in a dispenser unit for non-gaseous flowable material having a holder for the flowable material with an aperture therein, through which the material is transferred (in either direction) between an interior of the holder and an exterior of the holder, a flow meter for measuring flow of the material during transfer through the aperture, the flow meter having a sensor which determines a pressure difference between two locations within the holder which are vertically spaced from each other, one above and one below the surface of the material in the holder, at a number of different times during the transfer, the sensor emitting an output signal indicative of the flow from a calculation of the rate of change of the determined pressure difference.
By using the rate of change of pressure difference between above and below the material in the holder, only sequential pressure measurements, taken over relatively short time periods, are needed. This reduces the effect that any pressure sensor baseline drift may have on the determination of the material flow. Moreover, the sensor need only have a linear response over small pressure ranges.
Additionally, sine the flow determination depends on the pressure developed by a material above a sensor, the dimensions of the holder can be readily adapted so that small changes in the amount of material in the holder produce large pressure changes at the sensor at the base of the holder (i.e. by making the holder relatively long and thin).
Preferably, the holder is sized to provide a material-free space above the material within it when full, and the through-flow aperture, through which the material is transferred between the internal and the external of the holder, is provided in the base. By having the sensor measuring the pressure difference between the material-free space and a location proximate the aperture, the flow of substantially all of the material is metered.
Simply, the pressure sensor may be a differential pressure meter so that an output indicative of the pressure difference between the measurement locations within the holder is obtained quickly and directly. Alternatively, the pressure sensor may have two individual units placed to measure the pressure one above and one below the material in the holder. In this latter case, the meter would require adaptation to determine the difference between the two sensed pressures.
Most usefully the dispenser unit may be employed as part of a liquid anesthetic delivery system where it is typically used to dispense metered quantities of liquid having small flows.
According to a second aspect of the invention, there is provided an anaesthetic delivery system including a dispenser unit for non-gaseous anaesthetic material having an aperture connectable to supply of a carrier gas for delivering into a breathing gas a metered quantity of anaesthetic material, wherein the dispenser unit is a unit as described above wherein the meter of that unit is adapted to provide an output signal indicative of the flow of anaesthetic material from the holder. This output signal can be usefully employed in the control of a valve which is disposed to regulate the flow of the anaesthetic material into the carrier gas and which is operably connected to the meter to automatically regulate the flow in dependence of the output therefrom.